1. Field of the Invention
The invention relates to heat shrinkable, filled polymer compositions containing halogen-free flame retardant components suitable for use in electrical applications.
2. Description of the Related Art
Heat-shrinkable components, e.g., extruded or molded products, have been widely used in electrical applications. In a series of publications, Hoffman et al, disclose a range of uses in electrical insulation and environmental protection, from low voltage to high voltage installations. Insulation Enhancement with Heat-Shrinkable Components--J. W. Hoffman and others, IEEE Electrical Insulation Magazine, Part I, March/April 1991 Vol. 7, No.2, page 33; Part II, May/June 1991 Vol. 7, No.3, page 16; Part III, July/August 1991 Vol. 7, No.4, page 31.
Among the various requirements for satisfactory performance, these authors frequently emphasize the need for superior electrical insulation and prevention of moisture attack. The latter is accomplished either by the use of hydrophobic sealant materials or by the intentional incorporation of hydrophobic species in the heat-shrinkable compositions.
Another requirement specified for heat-shrinkable polymers is the requirement that they be semi-crystalline polymers. The most common class of semicrystalline polymers includes polyolefins. Therefore polyolefins have tended to be materials of choice for the manufacture of heat-shrink products. Polyolefin polymers containing additives such as flame retardants have been extensively disclosed in the prior art. See, for example, U.S. Pat. No. 4,174,343; JP 54,022,450; JP 54,007,451; JP 54,004,946; U.S. Pat. No. 3,810,862; EP 494,778; U.S. Pat. No. 5,116,891; U.S. Pat. No. 4,997,876; U.S. Pat. No. 4,871,795; U.S. Pat. No. 4,727,102 and U.S. Pat. No. 4,772,642. Flame retardants disclosed include such inorganic additives as magnesium hydroxide, calcium hydroxide, aluminum hydroxide, alkali phosphates and ammonium polyphosphate. Flame retardancy of these materials may be augmented using organic compounds including those containing halogen compounds. Other references disclose modified polyolefins. Modification is usually accomplished by the grafting of a selected functionality at intervals along the backbone of the polyolefin polymer. Suitable polymers in this category are generally acid modified polyolefins produced by reaction with, e.g., an unsaturated carboxylic acid or derivative thereof.
Japanese patent JP 1,108,241 discloses polyolefins grafted with a variety of acid species, including maleic acid, maleic anhydride, fumaric acid, acrylic acid etc. Flame retardant compositions, based on these modified polymers, are disclosed. The preferred additives used to impart flame retardancy include aluminum hydroxide, magnesium hydroxide, magnesium carbonate, calcium hydroxide, and the like. JP 2,284,940 discloses that the bending strength of a polyolefin modified with acrylic acid or maleic anhydride was approximately three times greater than the unmodified polymer. Flame retardant was added to the acid-modified polyolefin, the use of ammonium polyphosphate was not disclosed. Further, there was no teaching or suggestion that higher amounts of additives of this type could be accommodated.
Other references cite similar compositions, including JP 1,172440; JP 62,235,343; JP 62,011,745; U.S. Pat. No. 4,722,858. However, none of the modified polyolefin references disclose the use of ammonium polyphosphate containing compositions. There is no suggestion of the use of ammonium polyphosphate in compositions and structures used to provide electrical insulation and environmental protection, especially moisture protection.
The reason for the rejection of ammonium polyphosphate in electrically insulating or moisture repellant compositions is due to the fact that ammonium polyphosphate readily absorbs water. Thus it inherently attracts moisture which interferes with the effectiveness of electrical insulation and contact integrity at the site of an electrical connection. One reference, U.S. Pat. No. 4,772,642, attempts to solve the problem by encapsulating ammonium polyphosphate in a water-insoluble synthetic resin to produce a more water-stable fire-retardant composition. However, this is expensive to accomplish.
Similarly, U.S. Pat. No. 5,071,901 discloses the use of quaternary ammonium salts for modifying the surface of ammonium polyphosphate. This patent states that electrical insulation containing ammonium polyphosphate is generally deficient in water resistance. Therefore a surface treatment is needed to prevent leaching of ammonium polyphosphate from polymeric insulating materials, such as polyolefins, to which it may be added. It is stated that if leaching occurs, the electrical insulation and fire retardant performance will suffer.
The use of ammonium polyphosphate in heat shrinkable structures including tubes and molded shapes has been disclosed in U.S. Pat. No. 4,223,071. This patent discloses the use of ammonium polyphosphate as one of a series of phosphorus containing compounds used for erosion control of insulators for high voltage lines. No reference is made to its use in high performance insulation of electrical wiring or as a flame retardant.
The present inventor has discovered a combination of materials which will allow the use of ammonium polyphosphate in halogen-free flame retardant containing polymer compositions suitable for use where electrical insulation is of primary importance. In addition, the compositions of this invention can accommodate significantly higher quantities of flame retardant than that previously disclosed. The present invention provides compositions which take advantage of the superior flame retardancy of ammonium polyphosphate, while significantly reducing its tendency to attract water or water vapor. This is accomplished without any special surface treatment of the ammonium polyphosphate particles. Compositions of the current invention are therefore effective in preventing the common phenomenon of water leaching of ammonium polyphosphate from the polymer matrix. Additionally it has been discovered that flame retardancy is further enhanced by the incorporation of higher levels of flame retardant than was previously thought possible. This is accomplished without loss of mechanical performance of the polymer compositions.
These compositions are especially useful for flame retardant tape backings, such as those utilized in the electrical industry.
These compositions are also useful for flame retardant heat-shrink extruded or molded products for the electrical industry.